The manatee, often called a “sea cow,” is a large, fully aquatic mammal recognized for its gentle demeanor and slow movements. These herbivores graze on seagrass and other aquatic vegetation. Manatees prefer warm, shallow coastal waters, estuaries, and rivers, making their primary habitats tropical and subtropical regions. An adult West Indian manatee can measure up to 13 feet long and weigh over 3,000 pounds.
The Lack of Natural Predators
Healthy, adult manatees have almost no true natural predators. Their immense size and thick skin provide a substantial physical deterrent to most potential attackers. Furthermore, the manatee’s preference for shallow, near-shore waters, rivers, and brackish estuaries offers a geographical defense. These areas are generally inaccessible to large, deep-water marine predators like orcas or great white sharks.
Occasional interactions with large, opportunistic predators, such as the bull shark or American alligator, have been documented, but successful predation on a healthy adult is extremely rare. Bull sharks, which can tolerate both saltwater and freshwater, sometimes overlap with manatee territories in coastal areas. Alligators and crocodiles may share river systems, but manatees have been observed navigating around or even nudging these reptiles aside without conflict. Its environment and sheer bulk are its most effective defenses against traditional biological threats.
Vulnerability of Manatee Calves
While adults are largely safe, the smaller size of manatee calves makes them the most vulnerable to predation. A calf, which can weigh between 40 and 60 pounds at birth, lacks the bulk and experience of its mother. Calves stay close to their mothers for up to two years, relying on this maternal protection as their primary defense.
In areas where habitats overlap, young manatees may be targeted by large sharks, particularly the bull shark, which can venture into the shallow, murky waters preferred by manatees. Similarly, in freshwater rivers and spring-fed areas, large alligators or crocodiles may opportunistically attack a separated or weakened calf. Adult manatees will actively position themselves between a threat and their young, using their bodies to shield the calf from danger. Predation remains a relatively small cause of overall manatee mortality, but it is a genuine threat to the youngest members of the population.
Non-Predatory Causes of Manatee Deaths
Vessel collisions are the single largest known cause of mortality for manatees, especially in Florida. The slow-moving nature of the manatee and its habit of resting just below the surface place it directly in the path of fast-moving boats. These strikes often result in fatal blunt-force trauma from the hull or deep, debilitating cuts from propellers. Many living manatees bear distinct, parallel scars across their backs from these encounters. These injuries can impair their swimming or diving ability for life.
Manatees are highly susceptible to cold stress syndrome because they possess a low metabolic rate and minimal insulating body fat. They must seek waters consistently above 68 degrees Fahrenheit (20 degrees Celsius) to survive, often congregating near natural warm springs or power plant discharge canals during winter months. Prolonged exposure to colder water temperatures can lead to physiological shock, internal organ damage, and eventually death.
Harmful algal blooms, such as the “red tide” caused by the dinoflagellate Karenia brevis, are a significant threat. Manatees can inhale the airborne toxins or ingest them by consuming contaminated seagrass and filter feeders. This neurotoxic poisoning, known as brevetoxicosis, causes severe neurological and respiratory distress, leading to convulsions, loss of coordination, and mass mortality events.
Widespread loss of seagrass beds, the manatee’s main food source, has also contributed to starvation. Nutrient pollution from agricultural runoff and wastewater feeds large algal blooms that block sunlight, causing the destruction of these underwater meadows. This habitat degradation forces manatees to search for increasingly scarce food resources, weakening their overall health and making them more susceptible to other environmental stressors.